The alkylxanthine theophylline (compound A) a weak non-selective
adenosine antagonist (See Linden, J., et al., Cardiovascular Biology of Purines, eds. G. Burnstock, et al., 1998, pp 1-20) is useful therapeutically for the treatment of asthma. However, its use is associated with unpleasant side effects, such as insomnia and diuresis. In recent years the use of theophylline as a bronchodilator for relief of asthma has been supplanted by drugs of other classes, i.e., selective β2-adrenergic agonists, corticosteroids, and recently leukotriene antagonists. These compounds also have limitations and therefore the development of a theophylline-like drug with reduced side effects is still desirable.
It has been recognized that theophylline and its closely related analogue caffeine block endogenous adenosine acting as a local modulator of adenosine receptors in the brain and other organs at therapeutically useful doses. Adenosine activates four subtypes of G protein-coupled adenosine receptors (ARs), A1/A2A/A2B/A3. Enprofylline, (compound B), is another example of a xanthine
that has been reported to block A2B adenosine receptors and is used to treat asthma. However, this compound only weakly blocks A1, A2A and A3 adenosine receptors. It has also been shown by LaNoue et al (U.S. Pat. No. 6,060,481) that selective adenosine A2B antagonists are useful for improving insulin sensitivity in a patient.
It has been reported that therapeutic concentrations of theophylline or enprofylline block human A2B receptors, and it has been proposed that antagonists selective for this subtype may have potential use as antiasthmatic agents. (See Feoktistov, I., et al., Pharmacol. Rev. 1997, 49, 381-402; and Robeva, A. S., et al., Drug Dev. Res. 1996, 39, 243-252). Enprofylline has a reported Ki value of 7 μM and is somewhat selective in binding to human A2B ARs. (See Robeva, A. S., et al., Drug Dev. Res. 1996, 39, 243-252 and Linden, J., et al., Mol. Pharmacol. 1999, 56, 705-713). A2B ARs are expressed in some mast cells, such as the BR line of canine mastocytoma cells, which appear to be responsible for triggering acute Ca2+ mobilization and degranulation. (See Auchampach, J. A., et al., Mol. Pharmacol. 1997, 52, 846-860 and Forsyth, P., et al., Inflamm. Res. 1999, 48, 301-307). A2B ARs also trigger Ca2+ mobilization, and participate in a delayed IL8 release from human HMC-1 mast cells. Other functions associated with the A2B AR are the control of cell growth and gene expression, (See Neary, J., et al., Trends Neurosci. 1996, 19, 13-18) endothelial-dependent vasodilation (See Martin, P. L., et al., J. Pharmacol. Exp. Ther. 1993, 265, 248-253), and fluid secretion from intestinal epithelia. (See Strohmeier, G. R., et al., J. Biol. Chem. 1995, 270, 2387-2394). Adenosine acting through A2B ARs has also been reported to stimulate chloride permeability in cells expressing the cystic fibrosis transport regulator. (See Clancy, J. P., et al., Am. J. Physiol. 1999, 276, C361-C369.)
Recently Linden et al (U.S. Pat. No. 6,545,002) have described a new group of compounds and pharmaceutical compositions that are selective antagonists of A2B adenosine receptors (ARs).
Although adenosine receptor subtype-selective probes are available for the A1, A2A, and A3 ARs, only few selective antagonists and no selective agonists are known for the A2B receptor. Therefore, a continuing need exists for compounds that are selective A2B receptor antagonists.